It is known that optical lithography has reached the end of its capability at the 193 nm light wavelength and 1.35 numerical aperture (NA) immersion system. The minimum straight line resolution capability of this equipment is approximately 40 nm with an approximate 80 nm feature-to-feature pitch. A feature-to-feature pitch requirement lower than about 80 nm would require multiple patterning steps for a given structure type within a given chip level. Also, line end resolution becomes more challenging as lithography is pushed toward its resolution limits. One solution to line end shortening is to add a subsequent patterning step to cut features so as to form the line ends. Such line end cutting allows two line ends to be placed in closer proximity, and therefore may improve overall feature placement density, but at the cost of an additional patterning step. It should be understood that the added lithography steps for multiple patterning and/or line end cutting increases manufacturing cost, possibly to the point where any improvement in feature placement density is financially negated.
In semiconductor device layout, a typical metal line pitch at the 32 nm critical dimension is approximately 100 nm. In order to achieve the cost benefit of feature scaling, a scaling factor of 0.7 to 0.75 is desirable. The scaling factor of about 0.75 to reach the 22 nm critical dimension would require a metal line pitch of about 75 nm, which is below the capability of current single exposure lithography systems and technology.